Achromatic lens antenna



y 6, 1953 w. E. KOCK 2,640,154

ACHROMATI C LENS ANTENNA Filed Dec. 25, 1949 INVENTOR Ml E. KOCA ATTORNEY Patented May 26, 1953 Winston E. Kock, Basking Ridge, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, .N. Y., a corporation of NewYork Application December 23, 1949,?Serial No. 134,788

2 Claims.

This invention relates to electromagnetic wave lenses and more particularly to a wide frequency band metallic microwave lens.

My copending application, Serial .No. 642,723,

.filed January 22, 19%, and my article entitled Metal Lens Antennas, published in the .Pro- .ceedings of the I. R. .E., .vol. 34 :for November 1946, at ,pages 828 to 836, inclusive, both disclose in identical forma multiple zone electromagnetic wave or radio iphase-advance lens having a refractive index smaller than unityand comprising spacedparallel metallic walls. While :stepping thezones of the lens increases thefrequency band Width over which the action .of thelens is substantially uniform, as is taught in my abovementioned application and article and is illustrated by-the curves shown inFig. .17 of theaforesaid application and Fig. .13'1of the article, the increase in frequency :band widthis :not large.

To illustrate, referring to theaforesaid Fig. 17,

a six-zone unstepped metalliclens having am'aximum thickness of twelve wavelengths has a band of approximately 3 per cent. Stepping-the zones increases the band width to approximately '7 per cent, the increase being only *4 percent. In structures proportioned in accordance with the principles of the present invention, the band width is increased to a value in the order of per cent. In accordance :with the established practice .of those skillediin theart, the frequency band width of a device is expressed as the percentage obtained by dividing the width of "the handover which a given device per-iorms satisfactorily in cycles by the mid-frequencyof the band in cycles.

It is one object of this invention to provide electromagnetic wave lenses which will focus waves over a very large .or wide band of frequencies.

It is another :object of this invention to render an electromagnetic wave metallic phaseadvance lens achromatic. The meaning of .the term achromatic as applied to structures of the invention will be explained in detail below.

"It is still another object of this invention to obtain a simple inexpensive compound electromagnetic -waveachromatic lens of simple unitary construction.

In accordance with this invention, a concave converging, that positive, stepped multiple zone metallic electromagnetic wave lens section of the type disclosed in my aforesaidcopending application and-I. :R..Eaarticle and having a given frequency band width, is in effect combined -.With a convex diverging, that is, negative, unstepped multiple -zone metallic electromagnetic wave lens section having the same band width, to form a compound electromagnetic wave lens. Thecomipound lens comprises a plurality of spaced par- .allel metallic plates. One side or face .of the compound lens,corresponding to one side :of the negative lens section, is plane and unstepped; and the opposite side-or face of the compound ilens, corresponding to one side of the positive lens section, is concaveand stepped. Byreason of thefact that one lens section is stepped and the other section is notstepped, the dispersions of the two sections are substantially different .for each frequency of the electromagnetic waves .having frequencies within the broad frequency band to be transmitted through the lens structure whereby an achromatic electromagnetic wave-lens is obtained.

Ihe term achromatic is employed in optics to define a lens which :is free from color or which refracts white light energy Without decomposing it into its :constituent colors. .In optics-an achromatic lensis composed usually of twoglasses, a-convexand a concave,.theglasses being of substances having different refractive and dispersive powers. '(Quoted definitions. from Websteris Unabridgefitandard Dictionary.) As is well :known to those skilled in the .art, the variouscolors of lights are'merelydifferent wavelengths, or frequencies, of light energy, and an achromatic-optical lens is, therefore, simplya lens which produces substantially uniform refraction :over: a broad band of frequencies of light energy. -Aspointedout inthe introductory paragraphof my above-mentioned article, The extension of the useful .field of radio waves to the very short wavelengths has made the optical properties :of such waves increasingly apparent.

Inasmuch as the electromagnetic wave lensestof this invention are, in essence, composed of two portions, one 2portion being the equivalentof-an'optical convex-lens and-the other, the equivalent-of anoptical concave lens, .and the two iportions have dilierent'refractive and dispersivepowers which mutually compensate each other to produce substantially uniform refractive effectsover a broad band'of electromagnetic wave frequencies, the devicesof the present invention can .aptlybe termed achromatic electromagnetic wave compoundlenses. Likewise, .it appears entirel-y apt .to borrow the-terms positive .and negative from the opticalsart to denote converging and diverging.electromagnetic wave slenses. respectively, as .has Fbeen adoneihereinabove. Since 1 the lenses .of :the .present invention are phase advance lenses, i. e., they speed up the wave energy passing through them rather than delaying said energy as optical glass lenses do, the physical contours of the electromagnetic wave lenses of this invention, are the inverse of the physical contours of the optical lenses. For example, a positive, or converging, electromagnetic wave lens is concave while the corresponding optical glass lens is convex.

The invention will be more fully understood from the following specification taken in conjunction with the drawing on which like reference characters denote elements of similar function and on which:

Figs. 1 and 2 are, respectively, perspective and side views of a microwave system comprising a compound electromagnetic wave lens constructed in accordance with the invention.

Referring to Figs. 1 and 2, there is shown a translation device I, such as a transmitter or receiver, connected by a rectangular wave guide 2 to the throat aperture 3 of the sectoral horn 4. The long dimension of the mouth aperture 5 of the horn 4 is aligned with a focal line 6 of the cylindrical symmetrical compound electromagnetic wave lens I. The lens 1 has an axis or axial plane 8 and comprises a plurality of parallel identical metallic members 9 spaced apart a distance at least one-half wavelength, taken at the longest wavelength in the operating band, and held in position by means of the wooden frame Ill. Each pair of adjacent members 9 and the ai.. dielectric included therebetween constitute a channel II of the compound lens I. By reason of the spacing between the adjacent members 9, the refractive index of each channel, and hence of the compound electromagnetic wave lens I, smaller than unity and the phase velocity of the waves passing through the lens is advanced, as explained in my aforesaid copending application and in my above-mentioned article.

As shown in Fig. 2, each of the metallic plates 9 of the electromagnetic wave lens I comprises in effeet a section I2 having a stepped concave back side I3 and a concave front side I4, and a section I5 having a plane front side I6 and a convex back side I4, the concave front side of section I2 and the convex back side of section I5 being coincident. The circular line I4 does not necessarily represent a physical boundary between the abovedescribed portions of the structure since it will ordinarily be more convenient to construct each plate as a unitary member, but it does denote at least, the common effective boundary of the two adjacent sections I2 and I5. The plane side I6 of section I5 and the stepped concave side I3 of section I2 constitute, respectively, the front and back faces of the compound electromagnetic wave lens I. Since the shape of section I2 is concave-concavo, this section is, in accordance with the principles explained in my above-mentioned copending application and my I. R. E. article, a positive or converging lens section, whereas section I5 is, by the same principles, a negative or diverging lens section since the shape of this section is plano-convex. As shown on the drawing, the positive section I2 comprises the six stepped zones :1, b, c, d, e and j which have different elliptical curvatures, as explained in my aforesaid copending application and my above-mentioned article. Since the lens structure is vertically symmetrical about the axial plane 8, there will be two each of the stepped zones b, c, d, e, and j, as shown, symmetrically spaced above and below the zone a. The horizontal center line of zone a, of course, lies in plane 8 and zone a is also positioned symmetrically with respect to plane 8 as shown.

In operation, assuming device I is a receiver. electromagnetic waves having a frequency included in a given wide frequency band and having a plane wave front I1 and an incoming propagation direction I8, are propagated through the compound electromagnetic wave lens I and are focused on the mouth aperture 5 of the horn 4, as shown by the lines I9. As the frequency changes, the focal lengths of the positive and negative sections of the electromagnetic wave lens I vary oppositely substantially, as in the case of a compound achromatic optical glass lens when transmittinglight of various colors, 1. e., wavelengths or frequencies. It is because of this analogous action that the compound electromagnetic wave lens structuers of the present invention are so aptly designated achromatic." In other words, the change with frequency of the angle of convergence of a positive lens either electromagnetic wave or optical, is compensated to a substantial extent by an opposite change in the amount of divergence produced by the associated negative lens. This is obvious since the index of refraction n depends upon the wavelength as shown in Equation 3 of my abovementioned copending application or Equation 2 of my above-mentioned paper. This clearly means that if the wavelength is changed to a longer wavelength, n becomes smaller. 11 becoming smaller means that the refractive power of the lens becomes greater so that a positive or converging electromagnetic wave lens has greater convergence and a negative or diverging electromagnetic wave lens has greater divergence.

The change in divergence of a lens is equivalent to a change in the focal length and this can be correlated with the band width of the lens. This has been done in Fig. 17 of my above-mentioned application and in Fig. 13 of my abovementioned article. From either of these figures it is seen, for example, that the band width of an unstepped lens 6 wavelengths thick is the same as the band width of a stepped lens having 8 steps. Also, the rate of change of the refractive index with frequency, or so-called dispersion, of the positive electromagnetic wave lens is substantially different, over the given wide frequency band, from the dispersion for the associated neg-- ative lens section, by reason of the fact that the positive lens is stepped and its maximum thickness thus greatly reduced whereas the negative lens is not stepped and therefore has a. relatively much greater maximum thickness. Consequently, when the two portions of the lens are properly proportioned, i. e., to have the same band width as disclosed hereinabove as the frequency varies over the wide band, the focal length of the compound electromagnetic wave lens remains substantially constant, that is, the lens is achromatic. It may be noted incidentally that the addition of the negative lens to the positive lens increases the focal length of the combination over that of the positive lens taken alone, so that the compound lens, in one sense, corresponds to a single lens of longer focal length. It may also be added, by way of contrast, that in a compound optical lens the dispersions for the positive lens section and the negative lens sections are different not by reason of the fact that one section is stepped and the other is not, but by reason of the fact that the dielectric materials, usually lass, used in the two lens sections have different dispersion curves.

Although the invention has been explained in connection with a specific embodiment, it is not to be limited to the embodiment described inasmuch as other apparatus may be used in successfully practicing the invention.

What is claimed is:

1. A compound electromagnetic wave, phase advance, metallic lens for focusing waves included in a wide band of Wavelengths and having a given electric polarization, said lens comprising a plurality of metallic members extending parallel to said polarization and spaced apart a distance greater than one-half the longest wavelength in said band, said lens having a stepped concave face and a plane face, said concave and said plane sides being the surfaces which include and are defined by the stepped edges and the linear edges respectively of said plurality of metallic members, the contour line comprising the base line of the steps of said concave edge of each metallic member being a smooth curve of which the linear edge of said member is a chord, whereby said compound lens comprises, in efiect, a positive or converging multiple zone stepped electromagnetic wave, phase advance, metallic lens section and a negative or diverging multiple zone unstepped electromagnetic wave, phase advance, metallic lens section.

2. A compound electromagnetic wave, phase advance, metallic lens for focusing waves included in a wide band of wavelengths and which have a given linear electric polarization, said lens comprising a plurality of like flat conductive plate members arranged symmetrically in parallel planes with respect to a transverse axis normal to said plate members, said members being spaced at substantially equal intervals, the interval between each two successive plate members being greater than one-half wavelength of the lowest frequency of said wide band of wavelengths, each said plate member having two oppositely disposed longitudinal edges one of said longitudinal edges of each plate member beingstepped and concave, the other of said edges of each plate member being unstepped, the contour line comprising the base line of the steps of said stepped edge forming with said unstepped edge a portion bounded by an arc of a circle and a chord of said are, whereby said plurality of members form a compound lens comprising a positive, or converging, multiple zone, stepped, electromagnetic wave, phase advance, parallel plate, lens section combined with a negative, or diverging, multiple zone, unstepped, electromagnetic wave, phase advance, parallel plate, lens section, the compound lens so formed having substantially uniform refractive characteristics over a very broad band of frequencies or wavelengths.

WINSTON E. KOCK.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Darbord May 2, 1933 Bruce Dec. 10, 1946 Iams June 8, 1948 Kock July 31, 1951 OTHER REFERENCES Number 

